Valve body and Plater Guard question...

[OldPete]

Does anyone here know if on an American Standard single handle shower valve body:

Why do we want to cut the hole the size of the plaster guard instead of the size of the valve? Is there some kind of interference with the trimplate that requires this?

[plumber1]

The plaster guard also sets where the valve sits on the finished wall.

[Terry]

There my also be holes in the valve body that the trim needs.
I prefer the tile guy to cut to the plastic plaster guard.